Proper viscosity is of critical importance for petroleum based oils to work effectively, whether the oils is used for lubrication, or otherwise. Under normal service, oil will degrade, adversely affecting its viscosity due to factors such as oxidation, nitration, and particulate contamination. Even under normal operating conditions, viscosity can degrade at a rapid pace. The result of viscosity degradation is often excessive motor wear and damage. As such, testing the viscosity of oil should be an essential part of any maintenance program.
Many types of viscometers are known in the art. One type is the capillary tube viscometer which consists of a capillary tube and a fluid reservoir that holds a specified volume of sample liquid. The force of gravity produces a hydrostatic head in the reservoir, which causes the liquid to flow through the capillary tube. The time required for a fixed fluid volume to flow through the capillary tube is measured and the viscosity of the fluid is calculated based on the recorded time and the dimensions of the viscometer. The capillary-tube viscometer thus allows a user to derive viscosity from Poiseuille's Law for the flow of fluids through a capillary tube.
However, used oil can be difficult to properly monitor. Typical oil borne contaminants such as wear metals, soot, particulates, glycol and water can cause a conventional viscometer to clog or jam. This can result in erroneous viscosity readings. In addition, thick, opaque motor oil is often not compatible with the optical sensors used to monitor the flow of a sample in conventional automated equipment. A residue of oil can cover such sensors and result in a false signal. Since an essential part of such a test is determining the time required to empty the fluid reservoir, conventional viscometers are problematic when measuring used oil samples.
Accordingly, a need exists for better viscometers that can provide more reliable viscosity data for viscous, and often contaminate-filled, samples.